1. Field of the Invention
The present invention relates to a flexible photovoltaic device and improvement in the manufacturing method therefor.
2. Description of the Background Art
A flexible photovoltaic device and the manufacturing method thereof is described in Japanese Patent Laying-Open No. 1-105581, for example.
Referring to FIG. 1, a first resin layer 2 of polyimide resin with superior heat resistance is formed on a supporting substrate 1 such as of glass. The first resin layer 2 is transparent and insulating, as well as flexible. A transparent electrode 3, a thin film semiconductor layer 4 for photoelectric conversion, and a back metal electrode 5 are layered in sequence on the first resin layer 2. Semiconductor layer 4 comprises a p-i-n junction, a p-n junction or the like parallel to the film face. A second resin layer 14 such as of ethylene vinyl acetate copolymer (EVA) sheet is layered on back electrode 5. The second resin layer 14 is insulating and flexible.
Referring to FIG. 2, the photovoltaic device formed on the supporting substrate 1 is immersed in water 16 of a container 15 to be separated from the supporting substrate 1. This results in a flexible photovoltaic device shown in FIG. 3.
In the flexible photovoltaic device formed in the aforementioned manner, the second resin layer 14 is provided to protect the back metal electrode 5 from water 16. The second resin layer 14 is formed so as to have a shrinking force substantially equal to that of the first resin layer 2, to prevent the flexible photovoltaic device from curling after being separated from the supporting substrate 1.
In the flexible photovoltaic device obtained in such a manner, the layers having a weak bond with each other in the multilayered structure, i.e. the transparent electrode 3 and the semiconductor layer 4, are apt to be separated from each other at the edge of the photovoltaic device, due to mechanical stress such as bending or torsion exerted on the device. If the transparent electrode 3 and the semiconductor layer 4 are locally separated, the photoelectric conversion efficiency will be reduced significantly.
It is preferred that the first resin layer 2 has low adhesion to substrate 1 when the flexible photovoltaic device is separated from the supporting substrate 1. However, the first resin layer 2 must have sufficient adhesion to substrate 1 until the second resin layer 14 is formed. It generally takes a long time of two to three days for water 16 to permeate into the interface of the supporting substrate 1 and the first resin layer 2 to cause natural separation therebetween.
If an additional external force is applied to separate the photovoltaic device from the supporting substrate 1 in a short period, high stress will be generated in the region A surrounded by a broken line in FIG. 2, whereby the transparent electrode 3 and the semiconductor layer 4 are separated from each other locally to degrade the photoelectric conversion efficiency significantly.
Furthermore, because the first resin layer 2 of polyimide resin and the second resin layer 14 of EVA sheet have moisture permeability, local separation may be seen in the semiconductor layer 4 of the layered structure if the photovoltaic device is held in an environment of high humidity for a long time.